It is well known to provide hydrostatic ground drives for work machines such as agricultural combines and the like. Such drives typically include one or more fluid pumps operable for supplying pressurized fluid to one or more fluid motors in driving relation, respectively, to the drive wheels of the vehicle. Speed output of the drives is changed in a variety of ways. For example, a main and a supplemental pump arrangement can be provided, wherein use of the supplemental pump is triggered by demand for power or speed. As another example, the displacement of the pump or pumps and/or motor or motors is varied, for instance, under command of a propulsion handle or throttle. Engine speed can also be varied to increase or decrease pump output. Additionally, it is known to provide such drives in combination with multiple speed mechanical transmissions which can be shifted to provide discrete speed ranges.
Reference in the above regard, Burgart et al., U.S. Patent Publication No. 2007/0130931, which discloses hydraulic transmission controls with alternative operating modes wherein a swash plate of the transmission can be moved to any of a plurality of discrete positions based on the position of a manually operable switch, and wherein swash plate position can be varied to achieve different vehicle speeds.
Problems and shortcomings of known control systems for work machines such as combines include that a mechanical transmission may require operation in a low speed range for times of high torque requirements, such as field operation and the like, and transition to a higher speed range for road travel. The transition, particularly in heavily loaded machines such as agricultural combines, can take several seconds, making a transition to the higher speed range more difficult. Also, sometimes it may be desirable to change the combine speed range while the combine is stationary and on a slope, or subject to another condition or conditions, which would place the gear teeth of a mechanical transmission under loads that make shifts difficult. Additionally, control systems of combines typically utilize a multi-function handle movable by an operator in a forward range for varying forward speed, and movable in a reverse range for varying reverse speed, but some multi-function handles have a dead band about the neutral position which makes transition to that position non-linear and even rough and delayed.
What is sought therefore, is a control system for a hydrostatic ground drive for a work machine such as an agricultural combine, which provides selectable discrete speed ranges, but without requiring a mechanical transmission, which provides smooth, seamless transitions between the speed ranges, automatic speed adjustment for the new range, and which overcomes one or more of the problems and shortcomings set forth above.